What the children are able to do depends so much on their level of development, maturity, motor skills and attitude. It's hard to make any specific recommendations, but I'll outline a bit of what I did.
When my first son was three, we had just moved to our little "farm." There were obviously a good many chores he was simply too young to do, but I managed to find some. I would put an egg or two into a small basket and have him carry them to the house while I carried the rest of them. He dropped them a few times, of course, but soon he could be relied on to carry a good many of the eggs safely. He especially loved to carry the goose eggs. He was terrified of the geese (good thing), and was just SOOOO impressed that his Mum was both able and willing to take the gigantic eggs away from them.
And some things were given him (both sons in time) to do strictly for training purposes. The boy was obviously too young to do much that was useful, but not too young to learn. For example, he was given potatoes, a big pan of water, and a scrub brush. The chore was to scrub the potatoes, and he had a marvelous time with it always. Water, mud and physical action... perfect. He couldn't hurt the potatoes or himself, and gradually came to do that job well, very aware that those scrubbed potatoes (carrots, turnips, etc.) were then part of our dinner.
He was also given a wad of clean dishcloths and washrags. First I demonstrated how they were to be folded, and then helped him until he understood. He was then left to fold washrags and things himself, sitting up at the dining room table. Eventually, he got pretty good at folding clothes.
There are hundreds of things like that you can do with the very young, but it is a teaching thing and will take you as much or more time as doing the chore yourself. It's an investment of that time, of course, as later the children will be reliable to do these jobs themselves.
What you can do with older children who have not been taught much that way... hard to say. But you could start some of this teaching with chores appropriate to their age. Just give it to them in small bits. The command to "clean your room" means little to children and will only confuse them. Start with specific things, like pick up your clothes. Give them a big hamper or bin to put them in, then go through the clothing with them to sort the clean from the dirty or to be disposed of, using it as an opportunity to teach them one from another if they don't have a clear idea.
Then do the same with the toys. Sort the bin into what they want to keep and what can be discarded. My sister and I had lived with our aunt for a while after our father died. She didn't seem to care if we left our room a mess. But our mother was rather harsh in this and we soon learned to keep things picked up. She said that if we didn't care for something enough to put it away... then we didn't want it, and occasionally had it thrown out by the time we got home from school!!! Great incentive, even if there were many hurt feelings until we actually believed she was serious. Didn't take long.
The three year old won't benefit too much from the room cleaning example, of course... not enough sense of object permanence. Things really don't mean much to them in the abstract, only in the moment. The seven year old, however, is old enough and should have the judgment to benefit from this teaching. It will take longer to teach it now, since he was not trained in any of it earlier, I suspect.
The key is to be consistent, and to stick to your guns. If they find they can wheedle and manipulate you into letting them slack, it will be harder and harder to get them to do anything at all. Match ability first, then interest, but don't let their lack of interest stop you from teaching them to work. It may be the most valuable thing you can give them in the end.
Hope this helps.
